The present invention relates to an acoustic holography process and apparatus using a space-limited ultrasonic beam. It is used more particularly in the non-destructive testing of mechanical parts.
The principle of acoustic holography will be described relative to FIG. 1. By means of an ultrasonic transducer 2 able to transmit plane waves, one or more faults, i.e. one or more material breaks or interruptions in an object are insonified. In other words, the fault or faults are excited by an ultrasonic beam transmitted by the transducer and which is consequently called the "insonification beam". FIG. 1 shows the direction 4 of the ultrasonic beam, as well as the wave planes 5 perpendicular to said direction. The beam 6, diffracted and reflected by the fault or faults 3 is analyzed at several points 7, aligned in an examination plane 8, which can differ from the transmission plane 9 from which the insonification beam has been transmitted, by comparison with a reference beam similar to the insonification beam. This makes it possible to determine for each of the points 7 of examination plane 8, the phase and amplitude of the signal received at this point. The reconstitution or reconstruction of the fault or faults 3 is then carried out by calculating or reconstituting by any appropriate means, the sound pressure in the examined volume 10 containing the fault or faults 3. This pressure is produced by a transmitter located in examination plane 8 and which will have the predetermined amplitude and phase characteristics.
An acoustic holography process and apparatus are known and will be described with reference to FIGS. 2 and 3. Moreover, only acoustic holography in one plane, i.e. holography of planar sections of objects, is envisaged in FIGS. 2 and 3, the examination of a volume being obtained by successively effecting holograms in parallel section planes of said volume. FIG. 2 shows a planar section of a mechanical part 11, having a fault 12, the section plane coinciding with the plane of FIG. 2. The known apparatus, used for the acoustic holography of the fault, comprises a transmitter-receiver ultrasonic transducer 13, whose divergence angle .alpha. is very large and which is linearly displaceable on surface 14 of part 11, said surface being assumed flat. The transducer 13 occupies M successive positions P.sub.1, P.sub.2, . . . , P.sub.M, which are separated from one another by a distance at the most equal to the transmission wavelength of the transducer. The occupation of M successive positions by transducer 13 enables the acoustic beam transmitted by the latter to insonify the fault in several orientations, within the aperture L of the beam, which is equal to the distance separating the two extreme positions P.sub.1 and P.sub.M of transducer 13. For each of the M positions, the signal is transmitted by transducer 13, functioning as a transmitter, the echo signal reflected by the fault 12 being intercepted by said transducer then acting as a receiver and the phase and the amplitude of the echo signal are evaluated. Thus, it is the real part and the imaginary part of the echo signal (considered in complex form) which, in an equivalent manner, are evaluated. Thus, the amplitude-phase acoustic distribution on the surface 14 of part 11 is known. On the basis of these amplitudes and phases, the corresponding sound pressure is calculated (FIG. 3) on a circle, whose centre is the point O located on surface 14 of part 11 and in the centre of aperture L of the transducer and whose radius R is equal to the distance corresponding to the acoustic path necessary for reaching the fault or faults 12. On this circle, the sound pressure is at a maximum at the point or points forming part of the fault or faults 12.
The known process and apparatus described hereinbefore make it possible to obtain, substantially in real time, an evaluation of the size of the faults present in mechanical parts in a given direction, by means of a mechanical arrangement and relatively simple mathematical algorithms, but suffer from numerous disadvantages, the most important of which are given hereinafter.
Firstly, this process and this apparatus only give a very low signal/noise ratio. Thus, the ultrasonic beam transmitted by the transducer is very divergent and the acoustic pressure is proportional to z.sup.-2 .alpha..sup.-1, z being the distance between the transducer and the fault and .alpha. the solid divergence angle of said beam. Thus, the amplitude of the signal received by the transducer is a very rapidly decreasing function of the distance z and the solid angle .alpha., whilst the noise which is linked with the structure is a rising function of the solid angle .alpha..
Furthermore, this process and apparatus do not make it possible to obtain a good reconstitution of large flat faults or only give reconstitutions, whose quality is highly dependendent on the orientation of the studied faults. Thus, in the case of a large flat fault 15 in part 11, even if for a position P.sub.i of transducer 13 intermediate between the extreme positions P.sub.1 and P.sub.M, the mean insonification direction is perpendicular to fault 15 and encounters the latter in its centre (FIG. 4b), so that a large echo is received by this transducer, whilst the insonification of the fault on its edges, corresponding to the extreme transducer positions P.sub.1 and P.sub.M, gives a substantially zero echo signal on the transducer, said echo signal then being transmitted outside the insonification beam (FIGS. 4A and 4C).